The Hedgehog (Hh) pathway is a cell-cell communication system that plays important roles in development, regeneration and cancer. Hh signaling is orchestrated in vertebrates at primary cilia, antenna-like organelles that project fro the surfaces of most cells in our bodies and serve as signaling centers in development. Mutations in cilia genes cause a number of inherited human diseases called ciliopathies, many of which are characterized by birth defects and other phenotypes attributable to aberrant Hh signaling. Despite its importance as a target in cancer and regenerative medicine, many of the steps in Hh signaling remain poorly understood at the biochemical and cell biological level. My research program is focused on the major unsolved mechanistic questions in the vertebrate Hh pathway, with a particular emphasis on ciliary mechanisms that mediate signal propagation and transcriptional activation. The Hh signal is transmitted across the membrane by the 7-pass transmembrane protein Smo, the target for all anti-Hh drugs in clinical use. However, we do not understand the mechanism by which Smo is activated in response to Hh ligands, nor do we know how activated Smo in turn signals to the Glioblastoma (Gli) family of transcription factors. Primary cilia play an important role in both steps- Hh ligands promote the accumulation of Smo in the ciliary membrane, a critical step in signaling that eventually leads to the activation of Gl proteins as they traffic through the ciliary compartment. Delineating this mechanism will provide a valuable paradigm for how TM receptors relay signals from the ciliary membrane to the nucleus. Three major questions under investigation are (1) how Smo is activated by the main Hh receptor Patched 1 through endogenous small molecule ligands, (2) how activated Smo in the ciliary membrane transmits signals to the Gli proteins and (3) how Gli proteins are converted into transcriptional activators. We have made progress in each of these areas. Using new chemical tools to probe the interaction between Smo and oxysterols, endogenous lipids that can activate Hh signaling, we identified and structurally characterized a previously unknown ligand-binding site with regulatory potential in Smo. Using comparative proteomics, we identified a ciliary membrane protein complex that engages Smo at cilia in response to Hh signals and is required for Smo signaling. Finally, we have characterized both dynamic phosphorylation and protein association events that play critical role in Gli activation. Our work is supported by productive collaborations with investigators who have expertise in synthetic chemistry, structural biology, mass spectrometry and embryology. The successful completion of this project will provide (1) an answer to the question of Smo regulation, perhaps the longest-standing mystery in the Hh pathway, (2) an understanding of how ciliary protein trafficking drives signaling and how these processes are corrupted in ciliopathies, and (3) new strategies to monitor and modulate the pathway in Hh-related diseases.